The present invention concerns a method of re-alkalizing carbonated zones in concrete, and the like, in order to rehabilitate the condition of such concrete.
Reinforcement and other steel objects which are cast in concrete, cements or calcareous materials, such as mortar, plaster or gunite etc., are normally protected against corrosion because of the alkaline environment in the mass. Gradually however, the alkaline concentration is reduced by the absorption of carbon dioxide and sulphur trioxide. This absorption leads to a gradual neutralization of the alkaline environment. When the pH-value of the mass reaches 9.5 (approx.) the steel is no longer protected and corrosion is initiated.
The most well-known neutralization reaction is caused by the absorption of carbon dioxide, and is known as "carbonation".
Carbonation is already a serious problem today as is evidenced by the structural damage to buildings caused by the corrosion of reinforced steel, which in turn reduces the cross-section of the steel, and leads to the penetration of the concrete overlay as well as reductions in strength.
Damage caused by carbonation easily becomes a serious matter and is both difficult and expensive to repair. Traditionally this type of damage is repaired by chopping away the carbonated zone, sandblasting the exposed steel, and then concreting or using gunite. Sealing, combined with elutriation or a filler, is another approach. The result is seldom satisfactory as far as durability and load-carrying capability are concerned.
The purpose of the present invention is to find a means of repairing carbonated zones in concrete etc. which produce better results than the methods already in existence.
One of the characteristics of carbonation is that the damage is usually restricted to the surface zones of the structure, i.e., in the zone down to the first layer of reinforcement. This is normally a thin zone in relation to the remainder of the concrete cross-section, which is non-carbonated and has a surplus of alkaline matter.
The present invention concerns a method for the realkalization of carbonated zones in concrete by electrochemical migration of hydroxyl ions.
The carbonated zone is supplied with alkaline ions either from the fresh interior of the concrete etc., or from an external electrolytic medium by employing electricity. The re-alkalization from this alkaline matter re-establishes the corrosive protection of the steel.
The procedure is based on the following principle:
When a base electrolytic solution which contains calcium, sodium and potassium hydroxide, for example, is exposed to an electrical field between two electrodes, ions will be transported between the electrodes, which will make the area in the immediate vicinity of one of the electrodes highly alkaline. This alkaline content remains after the removal of the electrical field.
This procedure is executed in practice in the following way:
1. If the concrete etc., contains a double layer of reinforcement, where one layer is in carbonated concrete and the other is in fresh concrete, the reinforcement in the carbonated zone is connected to one pole on a rectifier or battery. While the structure is maintained under the electrical field, as described, the alkaline hydroxyl-ions migrate to the reinforcement pH conditions can be monitored by existing methods, such as pH-sensitive tracers or pH-electrodes. When the desired pH value is reached (usually over 12), the current can be disconnected.
2. If the concrete contains one layer of reinforcement, or possibly a second layer which is inaccessible, or if the distance between the two layers is too great for the technique described in point 1 to be feasible, an external electrode is located in a suitable electrolytic medium on the surface of the structure. This electrode can consist of wires, cords, plates, foil or sheet metal, conducting plastics or other conductive materials. The electrolyte can be an aqueous solution of calcium, sodium and/or calcium salts either in a liquid or absorbed in a porous medium such as rock wool, cellulose, sawdust, sand, clay and the like, or it can be strongly retarded concrete, mortar, cement grout, lime paste etc. When a cement-based concrete, mortar or paste is used, a strongly retarding substance such as sucrose is added to prevent the mass from setting, thus once the treatment is completed, this mass can be removed by scraping or flushing.
The external electrode is connected to one pole of the rectifier or battery, and the reinforcement in the carbonated zone is connected to the other pole in the manner described in point 1, above.
The speed of the re-alkalization process depends on the direct voltage which is applied, which is in turn dependent on the conductivity of the concrete and electrolyte, the density of the electrodes, and the distance between them. For normal building structures this voltage should be between 6-20 V, which will ensure that the re-alkalization is completed within a matter of days or weeks, depending on the overall conditions and the degree and depth of carbonation.